A common problem in the art of designing large multipass (3 or 4 pass) distillation trays and columns has been the problem of maintaining balanced ratios of liquid to gas throughput on each side of a multipass configuration. Typically the column itself is cylindrical; given a circular cross-section and chordal downcomers, it is generally accepted to design the areas on each side of the downcomers, which have different geometric shapes, to have the same areas or the same flow path length and generally similar perforations or other holes for gas throughput. If, during design, equal flow rates and/or treatment conditions such as liquid to vapor ratios are not achieved, which is commonly the case, the flow areas and/or height of the liquid in each tray area will be adjusted in order to achieve a balanced liquid to vapor ratio. The height and/or top grade of the liquid is commonly adjusted by adjusting the relative heights of the retaining weirs. Even though the same areas are usually involved, and the same number and size of orifices are provided for the passage of vapor, flows will vary considerably on different sides of the downcomers merely because the shapes of the areas will tend to affect the flow rates, which in turn affects the heights (depth) of liquid on the tray surface, which in turn affects the rate of gas or vapor throughput. Each of the effects mentioned on one side of a downcomer in turn affects the liquid to vapor ratio on the other side. Such a dynamic system is very difficult to control, much less anticipate, empirically, yet many distillation tower designers attempt to do so, most commonly by adjusting weir heights and shapes.
Various workers in the art have tried to control or anticipate the problems of imbalanced L/V ratios in different ways. See, for example, Keller's U.S. Pat. No. 3,729,179, which controls the horizontal flow patterns in a two-pass system by using a plurality of orifice-adjusting devices placed on the downcomer, together with vanes 19. Shobe, in U.S. Pat. No. 3,445,094, discloses a multipass system in which unequal areas are used on each side of the downcomer and baffles are provided for the downcomer to adjust the areas of the openings thereunder to be proportional to the unequal areas of the trays.
The reader may also be interested in U.S. Pat. No. 3,784,175, providing for downcomer openings which are adjustable according to fluctuations in fluid loads, and U.S. Pat. No. 4,036,918, which monitors overflash by a pressure differential at points near a downcomer and a weir.
Although the above methods of control are technically sound, they are unnecessarily complicated, and difficult to put into practice. Especially if the column is already constructed and in service.